Aiming at the problem of slowing down or refusing to operate the whole group of protection due to the decrease of component sensitivity when the bus ratio differential protection has drawn current in the fault in the area, this paper adopts a new bus protection method of the cosine of the angle between the virtual voltage difference of the fault component. In this method, by translating the position of the fault component voltage signal, and calculating the cosine of the included angle of the virtual voltage difference of the two fault components obtained by the translation, the internal and external faults are discriminated. When there is a fault in the area, the fault component virtual voltage difference is in the opposite direction, and the cosine value of the angle is negative; when the fault occurs outside the area, the direction of the fault component virtual voltage difference is the same, and the angle cosine value is positive. Finally, it is verified that the new method is not affected by the current drawn by the internal fault of the bus, and its action margin is at least 5.97 times that of the ratio differential protection through theoretical analysis and EMTP to build a model of the Laxiwa Hydropower Station. The parallelogram rule is also analyzed and simulated. It is verified that the new method is not affected by the high-impedance grounding of faults in the bus area; and the method has strong anti-current transformer saturation capability.